Practical use of an organic EL display apparatus employing a spontaneously light-emitting organic electroluminescent (EL) element is expected as a next-generation display apparatus, as it requires no backlight which is needed in a liquid crystal display apparatus, so as to be most suited for a reduction in apparatus thickness, and has an unlimited viewing angle. In the organic EL element employed in an organic EL display apparatus, the luminance of each light-emitting element is controlled by the current value that flows therein, and the organic EL element differs in this respect from the crystal liquid cell in which control is made by the voltage applied.
The active matrix method in a display apparatus with an organic EL element is an effective method over a passive matrix method in lengthening the life of an organic EL element and making a large-sized screen, and is being the subject of active research and development activities. Active matrix methods are grouped into a voltage writing method and a current writing method, depending on the type of a signal written into each pixel.
In an organic EL display apparatus according to the active matrix method, the brightness of each pixel is determined by the current value that flows to the organic EL element implemented in each pixel, and a control for the current value is performed by the voltage applied between the gate and the source electrodes of a drive transistor connected in series to the organic EL element. Generally, in many cases, the threshold voltage and mobility, the electric and physical characteristics of a drive transistor, vary in stability and uniformity among pixels, depending on the production process, material composition, and structure of the transistor. Therefore, reports are actively being made on the research to introduce a pixel compensation circuit and enhance the uniformity among the pixels.
With the above voltage writing method, compensation may be made only for the threshold voltage of the drive transistor, while with the above current writing method both the threshold voltage and the mobility may be compensated. In principle, the current writing method, capable of compensation for both the threshold voltage and the mobility, can easily realize display characteristics of high uniformity as compared with the voltage writing method.
A conventional pixel compensation circuit according to the current writing method is disclosed in patent document 1. FIGS. 50 and 51 show the conventional pixel compensation circuit according to the current writing method and the timing chart that represents the operation of the pixel compensation circuit, respectively. In this conventional technique, a second power line 109 is controlled as a scan line, and the diode characteristics of a light-emitting element 105 is utilized to provide on-off controls of the current flowing to the light-emitting element 105. The operation of this pixel compensation circuit will be described.
Immediately before the time period A shown in FIG. 51, the potential of the second power line 109 is made at least equal to or lower than the value of the potential of the first power line 108 plus the threshold voltage of the light-emitting element 105, so as to prevent current flow to and light emission by the light-emitting element 105. It is assumed here for the sake of simplicity that the potential of the first power line 108 is 0 volt, and the potential of the second power line 109 during the time period A is also 0 volt. Thereafter, the first scan line 107 is put in a high potential state, and the first switching element 101 and the second switching element 102 are turned on. All the transistors constituting the pixel circuit in FIG. 50 are assumingly of an n-channel type. In this instance, the signal line drive circuit 111 connected to the signal line 106 pulls a current signal Idata out of the signal line. Now that the first switching element 101 is in the on state, a connection is made between the signal line drive circuit 111 and the current retention unit 110 of the pixel 500, so that the current signal Idata flows from the second power line 109 to the signal line 106 via the driver element 103 and the first switching element 101. At this time, because the light-emitting element 105 is not applied with a forward voltage, no current flows thereto. Furthermore, as the second switching element 102 is in the on state, the voltages of the drain electrode and the source electrode of the driver element 103 are applied to both ends of the luminance signal retention capacitor 104. In other words, the potential difference Vds between the drain electrode and the source electrode is represented by the following equation (1).Vds=√{square root over ( )}(2Idata/β)+Vth  Equation (1)where β is a value proportional to the mobility of the driver element 103 and is represented by the following equation (2).β=μCox(W/L)  Equation (2)where μ is the mobility of the driver element 103, Cox is the gate oxide film capacity of the driver element 103, W is the channel width of the driver element 103, L is the channel length of the driver element 103, and Vth is the threshold voltage of the driver element 103.
Thereafter, if the first scan line 107 is turned to a low voltage state to turn off the first switching element 101 and the second switching element 102, and the second power line 109 is turned to a high voltage state such that the driver element 103 operates in the saturation region during the time period B, the current Ipix that flows to the light-emitting element 105 is made:Ipix=Idata  Equation (3)owing to the potential between the gate electrode and the source electrode of the driver element 103 that is being maintained at the value of the equation (1) by the luminance signal retention capacitor 104. Therefore, the current Ipix that flows to the light-emitting element 105 does not contain the characteristic values of β and Vth of the driver element 103. Accordingly, it becomes possible to compensate for variations in the mobility and threshold voltage of the driver element 103 as well as for variations in transistor geometries.    Patent document 1: Japanese Patent Application Publication No. 2003-195810 (page 21; FIGS. 5 and 7)